Axially Driven Piston/Cylinder Unit

ABSTRACT

An axially driven piston/cylinder unit comprising a cylinder, a piston being movable between first and second piston positions in the axial direction of the cylinder, a fluid bearing between the piston and the cylinder and defining a bearing surface on the piston side enclosing said piston over a part of the axial length of the piston, and a drive element mechanically-connected to the piston by means of a piston rod. The piston rod is embodied to permit and compensate for a radial offset, or an inclination between the cylinder axis and the longitudinal axis, defining the movement direction of the drive element. The piston rod is provided with a first drive-side joint section, the piston rod is also provided with a second piston-side joint section and that the second piston-side joint section is provided in the rear region of the piston, away from the piston crown.

The invention relates to an axially driven piston-cylinder unit and compressors comprising the same.

Such a piston-cylinder unit is disclosed in U.S. Pat. No. 5,525,845. This piston-cylinder unit of prior art comprises a piston driven by a linear drive, the piston being connected to the linear drive by means of a piston rod. This piston rod is rigid in the axial direction and is flexible in the lateral direction, i.e. in the radial direction. This design of the piston rod is intended to ensure that the piston is guided free from friction in the air bearing of the cylinder even when the drive axis does not run parallel with the cylinder axis. This unspecific flexible design of the piston rod may, however, lead to a situation where transverse forces act on the piston causing the piston to tilt in the cylinder or giving rise to a lateral offset of the piston axis relative to the cylinder axis. This results in asymmetries in the air gap between the outer circumference of the piston and the inner circumference of the cylinder, so that the fluid bearing is weakened in the region in which the distance between the outer piston circumference and the inner cylinder circumference is increased because the pressure of the bearing fluid is reduced at this point. However, this reducing pressure enables the fluid compressed in the cylinder volume to penetrate the bearing gap at this weakened point as soon as the fluid pressure increases, further expanding the bearing gap until the piston finally bears against the cylinder wall at the radially opposite point on the piston, causing undesirable friction.

The object of this invention is to further develop a generic piston-cylinder unit so that reliable operation of the fluid bearing, and hence reliable guidance of the piston in the cylinder, is guaranteed even when there is a lateral offset between the drive axis and the piston axis or where there is an inclination of both these axes relative to each other.

This object is achieved by the features specified in the claims.

The provision of the two joint sections in the piston rod first of all ensures that the piston rod is given the required flexibility, at defined points, to be able to compensate for a lateral offset of the axes. Because of the arrangement according to the invention of the piston-side joint of the piston rod in the rear region of the piston facing away from the piston crown, transverse forces acting on the piston are supported radially by the fluid bearing in the rear piston region, and away from the front piston-crown-side peripheral edge of the piston so that the fluid bearing is not influenced or is not substantially influenced by these harmful transverse forces. The risk that the piston may experience a lateral offset in the piston-cylinder unit of the invention due to transverse forces introduced into the piston by the piston rod in its front piston-crown-side region, which offset results in the state of the art in the detrimental weakening of the fluid bearing, is almost eliminated in the piston-cylinder unit according to the invention.

The second piston-side joint is preferably provided in the direction of the longitudinal axis of the piston at a point which lies level with the rear region of the piston-side bearing surface. This guarantees that any transverse forces introduced by the piston rod into the piston are directly supported at this point in the piston-side bearing surface on the fluid bearing.

Each joint section is preferably pivotable about at least one axis. However, it is also preferable for each joint section to be pivotable about two axes which are orthogonal to each other.

A particularly preferred design comprises joint sections which move in the manner of a ball and socket joint. This ensures that any offset relative to the radial direction between the drive axis and the cylinder axis can be compensated for without special alignment of the piston in the circumferential direction.

The fluid bearing preferably has a plurality of outlet nozzles for the fluid provided in the inner circumferential wall of the cylinder.

Here the outlet nozzles are arranged, in a particularly preferred embodiment, so that when the piston is in its second piston position, first outlet nozzles supply the front region of the piston-side bearing surface relative to the longitudinal extension of the piston, and second discharge nozzles supply the central or rear region of the piston-side bearing surface relative to the longitudinal piston extension, with pressure fluid.

If the outlet nozzles are provided in the front and rear regions of the piston-side bearing surface, an extremely uniform support of the piston over its longitudinal extension is achieved in the compression position of the piston. However, it is also advantageous for the first outlet nozzles to be provided in the front region and the second outlet nozzles to be provided in the central region, whereby the centre of gravity of the bearings extends forwards, i.e. towards the piston crown. Consequently a higher pressure is developed in the fluid bearing between the piston and cylinder in the region of the front end of the annular gap between the piston and cylinder, which pressure offers a higher resistance to the compressive pressure in the cylinder volume and is even better at preventing compressed pressure fluid from the cylinder volume from penetrating the bearing gap, even when a transverse force acts on the piston.

In a further optional embodiment the outlet nozzle are arranged so that when the piston is in its first piston position, the second outlet nozzles supply the front region of the piston-side bearing surface relative to the longitudinal piston extension with pressure fluid, and third outlet nozzles supply the rear region of the piston-side bearing surface relative to the longitudinal piston extension with pressure fluid. These optionally provided third outlet nozzles in the rear region can provide improved support of the piston in its retracted position, particularly during the action of a transverse force.

It is particularly preferable for the fluid bearing to be formed by a gas pressure bearing, the outlet nozzles being formed by gas outlet nozzles; an advantageous and particularly preferred embodiment is that of an air bearing.

A plurality of outlet nozzles preferably form nozzle devices.

The nozzle devices are preferably arranged annularly about the cylinder axis, preferably separated from each other in the axial direction of the piston-cylinder unit. As a result of this an extremely uniform fluid or gas cushion is developed between the piston and the cylinder.

For the formation of an extremely uniform fluid or gas cushion between the piston and the cylinder it is also advantageous for each nozzle ring to have a plurality of outlet nozzles uniformly separated from each other in the circumferential direction.

The outlet nozzles are preferably formed by micro holes drilled by means of an energy-rich jet, which bores are preferably of a conical design, their narrowest cross-section being located on the opening into the cylinder-side bearing surface. The micro holes produced in this manner generate a fluid or gas cushion of high uniformity and high load carrying capacity.

These micro holes are preferably drilled by means of a laser jet.

If the pressure fluid for supplying the outlet nozzles is derived from a fluid flow generated by compression of the cylinder volume, from the outlet duct, for example, a simple structure of the piston-cylinder unit can be achieved and at the same time an additional pressure generator for the pressure fluid for supplying the outlet nozzles may be dispensed with, thereby contributing to low cost production of such a piston-cylinder unit.

This piston-cylinder unit is particularly preferred when the piston is loaded by a moving part of a linear drive for the back and forth drive movement.

A particularly noteworthy and advantageous application of the piston-cylinder unit according to the invention takes place in a compressor for generating a pressure fluid, preferably in a linear compressor driven by a linear motor.

The invention is explained in detail in the following by way of an example with reference to the drawing, in which:

FIG. 1 shows a piston-cylinder unit according to the invention with the piston in a retracted position;

FIG. 2 shows the same piston-cylinder unit with the piston in the vicinity of the compression position.

FIG. 1 shows a longitudinal section through a piston-cylinder unit 1 with a cylinder 2 and a piston 3. Cylinder 2 is provided with a cylinder bore 10 in which piston 3 is accommodated so that it can be displaced back and forth in the direction of the longitudinal axis X of cylinder bore 10 and is freely guided. The head-side end wall 12 of cylinder bore, formed on a cylinder head 23, inner circumferential wall 14 of cylinder bore 10 and piston crown 16 delimit cylinder volume 18.

An inlet duct 22 provided with a valve 20, shown diagrammatically, opens into head-side end wall 12 of cylinder bore 10. In head-side end wall 12 is also arranged an outlet duct 24, which has a corresponding valve 26; this outlet duct also opens into cylinder bore 10.

During a movement of piston 3 to the left, shown in FIG. 2, fluid is sucked through inlet duct 12 and inlet valve 20 into cylindrical space 18, and during a movement of piston 3 to the right, this fluid is expelled in the compressed state through outlet valve 26 and outlet duct 24. Piston-cylinder unit 1 shown is part of a piston working machine in which the expelled fluid is gaseous, as is the case with a compressor. In principle, however, the invention may also be used in other piston working machines such as pumps.

Some of the expelled gaseous fluid is conveyed out of outlet duct 24 through a connecting duct 28, which is provided in cylinder head 23 and housing 21 of cylinder 2 and fed into annular ducts 30, 32, 34, which are also provided in housing 21 of cylinder 2, and which surround cylinder bore 10 annularly. Annular ducts 30, 32, 34 are separated from each other in the direction of longitudinal axis X of cylinder bore 10. Each of annular ducts 30, 32, 34 is provided with a multiplicity of micro holes 30′, 32′, 34′ which, distributed uniformly around the circumference of cylinder bore 10, connect each annular duct 30, 32, 34 to the inside of cylinder bore 10 and in doing so penetrate inner wall 14 of the cylinder. Micro holes 30′, 32′, 34′ of each annular duct 30, 23, 34 therefore form a corresponding annular nozzle arrangement 30″, 32″, 34″. Compressed gas, which is conveyed through connecting duct 28 into annular ducts 30, 32, 34 can therefore escape through micro holes 30′, 32′, 34′ and form a gas cushion laterally supporting the piston between a cylinder-side bearing surface 15 on the inner circumferential wall 14 of cylinder 2 and a piston-side bearing surface 38 on the outer circumferential wall 36 of piston 3.

First annular duct 30, with micro holes 30′ assigned to it, is located in a region in which the piston only covers micro holes 30′ when it is in the vicinity of the compression position, i.e. when cylinder volume 18 is minimised, as shown in FIG. 2. In this case piston 3 covers the front, first micro holes 30′ with bearing surface 38 in front region 3″.

In the position shown in FIG. 1, in which cylinder volume 18 is at a maximum, micro holes 30′ do not contribute to the formation of a gas cushion between inner circumferential wall 14 of cylinder 2 and outer circumferential wall 36 of the piston. Because of the extremely small cross-section of micro holes 30′, however, the pressure loss that therefore occurs is not serious. However, a valve arrangement (not shown). which loads first annular duct 30 with compressed gas when piston 3 covers micro holes 30′, may also be provided.

Second annular duct 32 is arranged so that micro holes 32′ assigned to it are always covered by piston 3, so that micro holes 32′ contribute to the formation of the gas cushion between inner circumferential wall 14 of cylinder 2 and outer circumferential wall 36 of piston 3 throughout the axial path of movement of piston 3.

Third annular duct 34 is furthest away from head-side end wall 12 of cylinder bore 10. Micro holes 34′, assigned to third annular duct 34, are therefore not covered by piston 3, i.e. by bearing surface 38 in rear region 3′ of the piston, until piston 3 is located in the region of its retracted position in which cylinder volume 18 is at a maximum. The provision of third annular channel 34 with micro holes 34′ assigned to it is optional and only serves to improve the running properties of piston 3 in cylinder bore 10.

Further similarly constructed annular nozzle arrangements in inner wall 14 of cylinder bore 10 may be provided between annular ducts 30, 32, 34 with micro holes 30′, 32′, 34′ assigned to them, which holes each form the annular nozzle devices 30″, 32″, 34″.

Piston 3 is driven by drive element 50 of a linear drive 5 that is longitudinally displaceable back and forth along an axis Y, in a vibrating manner, which drive is only represented diagrammatically in the figure. Moving drive element 50 is connected mechanically to piston 3 by means of a piston rod 4. Piston rod 4 is non-elastic in the axial direction and is therefore capable of transmitting axial forces from drive element 50 to piston 3. This force transmission presents no problems if longitudinal axis Y of drive element 50 and longitudinal axis X′ of piston 3 and longitudinal axis X of cylinder 2 are identical.

Where linear drive 5 is not aligned exactly with piston-cylinder unit 1, longitudinal axis X of drive element 50 can be inclined to longitudinal axis X of cylinder 2 or offset parallel with it. This means that axis X′ of piston 3 is not aligned exactly with axis X of cylinder 2 either, so that according to the state of the art piston 3 is positioned slightly obliquely in cylinder 2, thus giving rise to contact between the piston and cylinder, which under certain circumstances cannot even be supported by gas pressure bearing.

For this reason piston rod 4 is provided with a first drive-side joint section 40 and a second piston-side joint section 42. In the example shown, these joint sections 40, 42 are designed as sections with a diameter that is reduced relative to the remaining piston rod sections. Piston rod 4 is therefore more flexible in joint sections 40, 42 than in the remaining piston rod sections, with the result that they are able to be bent into joint sections 40, 42. Therefore, if axes Y and X are misaligned, piston rod 4 compensates for the angular offset of these two axes relative to each other or the lateral offset of these two axes relative to each other, denoted in the figures by d, which means that longitudinal axis X′ of piston 3 is aligned essentially with axis X of the cylinder. In this case small transverse forces are introduced into the piston, which forces act essentially perpendicularly to axis X′ of piston 3 and can be supported by the gas cushion formed between cylinder-side bearing surface 15 and piston-side bearing surface 38.

Piston-side joint section 42 of piston rod 4 is arranged in rear region 3′ of piston 3. The rear region is here defined as the region facing away from piston crown 16 with respect to a central plane M situated orthogonally on piston-side bearing surface 38. Front piston region 3″ is therefore that region between central plane M and the front, piston-crown-side end of piston 3.

Since the above-mentioned lateral forces act from piston rod 4 in the region of joint section 42 orthogonally to longitudinal piston axis X′, they are supported by the section of piston-side bearing surface 38 located in this region against the gas cushion and hence against cylinder-side bearing surface 15. If in this case there is a slight deformation of the gas cushion, i.e. displacement of the annular space formed between piston-side bearing surface 38 and cylinder-side bearing surface 15, this deformation takes place essentially locally in rear region 3′ of piston 3 without exerting any substantial effect in front region 3″ of piston 3. The risk that compressed gas escapes from cylinder volume 80 and enters bearing gap asymmetrically due to such a deformation of the annular bearing gap between piston 3 and cylinder 2 in front region 3″ of piston 3, and may then slide or tip the piston to the side, is therefore extremely small.

The design of the axially driven piston-cylinder unit according to the invention provides improved guidance of piston 3 in cylinder 2 due to the special position of piston-side joint section 42 in rear piston region 3′, and results in a high degree of operational reliability. The front, first nozzle arrangement 30″ reinforces this higher degree of reliability by strengthening the gas cushion formed by the fluid bearing at this point in the compressed condition of the piston-cylinder unit.

The invention is not limited to the above exemplary embodiment, which serves merely as a general explanation of the core concept of the invention. Within the scope of protection the device according to the invention may instead assume embodiments other than those described above. In this case the device may, in particular, have features which represent a combination of the individual features described in the claims.

Reference symbols used in the claims, description and drawings serve merely to ensure a better understanding of the invention and will not limit the scope of protection. 

1-17. (canceled)
 18. An axially driven piston-cylinder unit comprising: a cylinder; a piston which can be displaced back and forth in the axial direction of the cylinder between a first piston position, in which the cylinder volume enclosed by the piston and the cylinder is at a maximum, and a second piston position, in which this cylinder volume is at a minimum; a fluid bearing provided between the piston and the cylinder, which bearing supports the piston in the cylinder so that it can be displaced axially and which determines a piston-side bearing surface surrounding the circumference of the piston, at least over a part of the axial extension of the piston; a drive element that can be displaced back and forth in the axial direction of the cylinder or essentially in parallel with it, which element is connected mechanically to the piston by means of a piston rod; wherein the piston rod is designed so that it permits, and compensates for, a radial offset or an inclination between the cylinder axis and the longitudinal axis determining the direction of movement of the drive element; wherein the piston rod is provided with a first drive-side joint section, that the piston rod is provided with a second piston-side joint section and that the second piston-side joint section is provided in the rear region of the piston facing away from the piston crown.
 19. The axially driven piston-cylinder unit according to claim 18, wherein the second piston-side joint section is provided in the direction of the longitudinal axis of the piston at a point which is situated level with the rear region of the piston-side bearing surface.
 20. The axially driven piston-cylinder unit according to claim 18, wherein each joint section can be pivoted about at least one axis.
 21. The axially driven piston-cylinder unit according to claim 20, wherein each joint section can be pivoted about two axes which lie orthogonally relative to each other.
 22. The axially driven piston-cylinder unit according to claim 20, wherein each joint section has the displaceability of a ball and socket joint.
 23. The axially driven piston-cylinder unit according to claim 18, wherein the fluid bearing has a plurality of outlet nozzles for the fluid provided in the inner circumferential wall of the cylinder.
 24. The axially driven piston-cylinder unit according to claim 18, wherein the outlet nozzles are arranged so that when the piston is in its second piston position, first outlet nozzles supply the front region of the piston-side bearing surface relative to the longitudinal piston extension, and second outlet nozzles supply the central or rear region of the piston-side bearing surface relative to the longitudinal piston extension, with pressure fluid.
 25. The axially driven piston-cylinder unit according to claim 18, wherein the outlet nozzles are arranged so that when the piston is in its first piston position, the second outlet nozzles supply the front region of the piston-side bearing surface relative to the longitudinal piston extension, and third outlet nozzles supply the rear region of the piston-side bearing surface relative to the longitudinal extension, with pressure fluid.
 26. The axially driven piston-cylinder unit according to claim 18, wherein the fluid bearing is formed by a gas pressure bearing, preferably an air bearing, wherein the outlet nozzles are formed by gas outlet nozzles.
 27. The axially driven piston-cylinder unit according to claim 18, wherein a plurality of outlet nozzles each form nozzle devices.
 28. The axially driven piston-cylinder unit according to claim 27, wherein the nozzle devices are separated from each other in the axial direction of the piston-cylinder unit, and are preferably designed annularly about the cylinder axis.
 29. The axially driven piston-cylinder unit according to claim 27, wherein each nozzle device has a plurality of outlet nozzles separated from each uniformly in the circumferential direction.
 30. The axially driven piston-cylinder unit according to claim 18, wherein the outlet nozzles are formed by micro holes drilled by means of an energy-rich jet, which holes are preferably of a conical design, wherein their narrowest cross-section is located on the opening into the cylinder-side bearing surface.
 31. The axially driven piston-cylinder unit according to claim 30, wherein the micro holes are drilled by means of a laser jet.
 32. The axially driven piston-cylinder unit according to claim 18, wherein the pressure fluid for supplying the outlet nozzles is derived from a fluid flow compressed by compression of the cylinder volume.
 33. The axially driven piston-cylinder unit according to claim 18, wherein the piston is loaded by a displaceable drive element of a linear drive for the drive moving back and forth.
 34. A compressor for generating a pressure fluid and comprising: an axially driven piston-cylinder unit including: a cylinder; a piston which can be displaced back and forth in the axial direction of the cylinder between a first piston position, in which the cylinder volume enclosed by the piston and the cylinder is at a maximum, and a second piston position, in which this cylinder volume is at a minimum; a fluid bearing provided between the piston and the cylinder, which bearing supports the piston in the cylinder so that it can be displaced axially and which determines a piston-side bearing surface surrounding the circumference of the piston, at least over a part of the axial extension of the piston; a drive element that can be displaced back and forth in the axial direction of the cylinder or essentially in parallel with it, which element is connected mechanically to the piston by means of a piston rod; wherein the piston rod is designed so that it permits, and compensates for, a radial offset or an inclination between the cylinder axis and the longitudinal axis determining the direction of movement of the drive element; wherein the piston rod is provided with a first drive-side joint section, that the piston rod is provided with a second piston-side joint section and that the second piston-side joint section is provided in the rear region of the piston facing away from the piston crown. 